System and method for additive manufacturing

ABSTRACT

A method for forming a component includes providing a first layer of a mixture of first and second powders. The method includes determining the frequency of an alternating magnetic field to induce eddy currents sufficient to bulk heat only one of the first and second powders. The alternating magnetic field is applied at the determined frequency to a portion of the first layer of the mixture using a flux concentrator. Exposure to the magnetic field changes the phase of at least a portion of the first powder to liquid. The liquid portion couples to at least some of the second powder and subsequently solidifies to provide a composite component.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No. 16/202,453, filed on Nov. 28, 2018, which claims the benefit of U.S. patent application Ser. No. 14/190,460, filed on Feb. 26, 2014 now abandoned, which claims the benefit of U.S. Provisional Application No. 61/833,020 filed on Jun. 10, 2013, U.S. Provisional Application No. 61/868,625 filed on Aug. 22, 2013, U.S. Provisional Application No. 61/885,806 filed on Oct. 2, 2013, U.S. Provisional Application No. 61/896,896 filed on Oct. 29, 2013, U.S. Provisional Application No. 61/898,054 filed on Oct. 31, 2013, and U.S. Provisional Application No. 61/938,881 filed on Feb. 12, 2014. The entire disclosures of each of the above applications are incorporated herein by reference.

FIELD

The present disclosure relates to a system for additive manufacturing and, more particularly, to a system and method of selectively sintering a mixture of powders using micro-induction sintering.

BACKGROUND

This section provides background information related to the present disclosure which is not necessarily prior art.

Current processes for producing high purity bi-component materials, such as refractory metal parts, include powder and ingot metallurgy. The ingot metallurgy process begins with selecting and blending suitable powders, pressing into bars, and sintering. An electron beam or plasma or arc furnace is used to melt the bar in an inert atmosphere and cool it into an ingot. The melting can be done in multiple steps. Electron beam melting and re-melting removes impurities to produce an essentially pure ingot. The ingot is thermo-mechanically processed and further cold or hot worked as needed (or cold worked with intermediate annealing) to produce a desired shape such as plate, sheet, rod or fabricated. Components may also be machined directly from ingots.

The sintering process consumes a significant amount of furnace time, but it is required to provide sufficient mechanical strength in the bars and is a preliminary deoxidation step for the refractory metal powder, such as tantalum. The bars are usually electron beam-melted under a hard vacuum to remove impurities. The electron beam melting process can also consume a significant amount of furnace time and power.

Laser additive manufacturing is a direct deposition process that uses a high power laser and powder feeding system to produce complex three-dimensional components from metal powders. The high power laser and multi-axis positioning system work directly from a CAD file to build up the component using a suitable metal powder. This process is similar to conventional rapid prototyping techniques such as stereolithography, selective laser sintering (SLS), and laser welding. Laser welding was developed to join two components or to fabricate an article integral to a component. Such a laser process has been used to manufacture near-net shape titanium components for the aerospace industry.

To date, an additive manufacturing process does not exist for higher temperature bi-component refractory and tooling materials, or bi-materials, where one material is sensitive to the high energy applied by the laser. The application of a directed high energy beam to a powder mixture can cause damage to one or more of its constituent components. In this regard, this energy can cause undesired phase and structural changes within one or both of these component materials. As an example, superconductors encapsulated into a metal matrix are highly sensitive to the application of a laser induced energy which may destroy their superconducting capabilities. Additional problems can occur when the application of a laser to a powder mixture leads to undesired chemical reactions between the materials. As such, there is a need for an additive manufacturing system which overcomes some of the deficiencies listed above and allows for a more creative combination of materials.

SUMMARY

This section provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or all of its features.

A method for forming a component includes providing a first layer of a mixture of first and second powders. The method includes determining the frequency of an alternating magnetic field to induce eddy currents sufficient to bulk heat only one of the first and second powders. The alternating magnetic field is applied at the determined frequency to a portion of the first layer of the mixture using a flux concentrator. Exposure to the magnetic field changes the phase of at least a portion of the first powder to liquid. The liquid portion couples to at least some of the second powder and subsequently solidifies to provide a composite component.

According to the present teachings, a method for forming a component is presented. The method includes selectively applying a magnetic field to a first portion of a layer of a powder mixture to selectively melt the first portion.

According to further teachings, a method of forming a component from a mixture of first and second particles is presented. The method includes selectively applying a magnetic field to a first portion of the powder mixture. The magnetic field is applied at a frequency and field strength to cause melting of the first particles within the first portion of the powder.

According to another teaching in the present disclosure, a method for forming a component is presented. The method includes providing a layer of a mixture of first and second particles, the first particles having a first particle size distribution. The method includes applying a high frequency magnetic field to a first portion of the layer. The high frequency magnetic field has a plurality of frequencies between a first frequency corresponding to a first portion of the first size distribution, and a second frequency corresponding to a second portion of the size distribution.

According to another teaching in the present disclosure, a method of forming a component from a mixture of first and second powder materials is disclosed. The first powder material has a first resistivity and the second material has a different second resistivity. The method includes applying a high frequency magnetic field to a first portion of the powder mixture so as to cause at least a portion of the particles in the first powder material to melt.

According to another teaching of the present disclosure, a method of forming a component from a mixture of first and second powder materials is disclosed. The method includes forming a first layer of a material of the mixture of particles. Next, a magnetic field is applied to a first portion of the first layer so as to cause a first set of particles in the first portion to melt. Next, a second layer of the mixture of particles is disposed over and in contact with the first layer. A second magnetic field is selectively applied to a second portion of the second layer to cause a second set of particles in the second layer to melt. When the sintering is completed, the first portion is coupled to the second portion.

According to another teaching, the method above includes applying a plurality of magnetic fields having between a first frequency and first power level, and a second frequency and second power level to the first portion of the first layer so as to effect the melting of powder particles having one of varying size and resistivity.

According to another teaching, a system for forming a component is provided. The system includes a bed configured to hold a first layer of a mixture of metal powder, and a magnetic flux concentrator configured to apply a magnetic field at a frequency and field strength necessary to melt a first portion of the powder within the first layer.

The system further includes a mechanism for applying a second layer of a second mixture of material in contact with the first layer. The system then applies a second magnetic field to the second layer to melt a second portion of the second layer, where the second portion is fused to the first layer when the second magnetic field is removed. Further areas of applicability will become apparent from the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.

DRAWINGS

The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure.

FIG. 1 represents a schematic representation of the additive manufacturing system according to the present teachings;

FIGS. 2a-2d represent the application of micro induction heating to particles according to the present teachings;

FIG. 3 represents a graph representing power transfer factors for materials subjected to micro induction heating;

FIGS. 4a-5c represent micro induction sintering according to the present teachings;

FIGS. 6a-6c represent graphs showing operating parameters for the micro induction heating system according to the present teachings;

FIGS. 7a-7c represent the formation of a fiber reinforced metal matrix material according to the present teachings;

FIG. 8 represents fibers which can be used in the process shown in FIGS. 7a -7 c;

FIG. 9 represents the simulated application of high frequency magnetic fields to powders having materials having varying size distributions;

FIG. 10 represents a flow chart describing the formation of a laminar component;

FIGS. 11a-11c represent the formation of a bi-material component according to the present teachings;

FIG. 12 represents a flux concentrator according to the present teachings; and

Tables 1-5 represent various material properties of powder materials used according to the present teachings.

Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings.

DETAILED DESCRIPTION

Example embodiments will now be described more fully with reference to the accompanying drawings.

FIG. 1 depicts a system 10 for producing a component 11 using an additive micro-inductive sintering method in which layers of a mixture of powder 12 are consolidated. A powder holding tray or bed 13 retains the mixture of powder 12. Disposed above the tray 13 is a gantry 15 which supports a magnetic flux concentrator 17. A power supply and wave form generator 19 provides energy to magnetic flux concentrator 17 to apply a distinct, high frequency alternating magnetic field to selectively heat individual particles of the mixture of powder 12.

System 10 further includes a mechanism 21 having a pouring spout and leveling mechanism that recursively places layers of the mixture of powder 12 over previously consolidated portions of the mixture of powder 12. Also shown is a sensor 25 that detects information such as the transfer of energy to the mixture of powder and the degree of consolidation.

Unlike laser or electron beam based additive manufacturing techniques in which the metal powder is heated indiscriminately by an external energy source, the system 10 uses micro-induction sintering for the selective heating of individual particles by tailoring the frequency of an applied magnetic field. During micro-induction sintering, the system 10 applies a localized high frequency magnetic field produced over an upper surface of the powder bed using the flux concentrator 17. System 10 causes a rapid heating of individual particles followed by a rapid cooling of the consolidated material due to a decoupling of the high frequency magnetic field from the melted particles that no longer exhibit the particle size being excited.

Heating of metallic particles within the mixture of powders 12 by induction is a result of both Joule heating due to eddy currents in non-magnetic metallic particles and hysteresis loss in magnetic particles, both of which result from the application of a high frequency magnetic field. For non-magnetic metals, eddy currents flow within a certain distance from the surface of the material.

$\begin{matrix} {\delta = \sqrt{\frac{\rho}{\pi f\mu}}} & \lbrack 1\rbrack \end{matrix}$

The distance within the metal at which the eddy current is reduced to approximately 37% of the value at the surface is called the skin depth δ and can be written as where ρ is the resistivity and μ is the permeability of the material, and f is the frequency of the magnetic field. In order to completely heat a metal particle by induction, the particle is immersed in a high frequency magnetic field such that the skin depth is approximately one half the diameter of the particle. Generally, high power transfer to the particle occurs near a diameter approximately four times the skin depth for simple geometries such as plates and cylinders with the magnetic field parallel to the axis of the part. For spheres, it is expected this ratio of the particle diameter to the skin depth would be higher.

FIG. 2a depicts the heating of a single particle by induction. The diameter of the particle is approximately 2δ. In this case, the eddy currents penetrate deep into the particle and bulk heating of the entire particle occurs by induction and heat transfer through the particle at a single frequency. Due to particle size distributions as well as particle shape anomalies, a band of frequencies is preferred to sinter a mixture of powders 12.

FIG. 2b depicts when the diameter of the particle is much larger than 8. Due to the directional nature of the magnetic field, only a portion of the particle outer skin is melted corresponding to the skin depth δ. For given resistivity and particle sizes, the melting can occur either only at the surface or through an entire circular layer of the particle (see FIG. 2c ). A band of frequencies can be applied to correspond to various hemispherical diameters D₁-D₅ at different circles of the sphere. In the examples depicted in FIGS. 2b and 2c , the frequencies applied can vary from 1 to about 5 times the frequency calculated to melt the largest diameter for a given particle having a specific resistivity.

FIG. 2c depicts when a band of frequencies are applied to melt a set of cylindrical disks through the particle. In this example, D₁-D₅ correspond to frequencies which form a skin depth of approximately 2 δ. Optionally, to melt the particle, the frequency band of the magnetic field F need not completely cover each of the frequencies corresponding to diameters D₁-D₅. Melting of the whole or a sufficient portion of the particle can occur by applying frequencies corresponding to skin depths, for diameters D₂ and/or D₃, where melting the entire particle, or surface of the particle occurs through heat conduction. The heat energy required to melt the remainder of the particle transfers through the particle via normal heat diffusion processes.

In FIG. 2d , the skin depth is much larger than the diameter of the particle and the eddy currents largely cancel in the particle. In this case, the particle does not couple well to the alternating magnetic field and the material absorbs very little power. It is envisioned the system would use frequencies such that the heating would be completed as shown in FIGS. 2a-2c . There is little heating of the particle at frequencies depicted in the case shown in FIG. 2 d.

For simple shaped (e.g. flat or cylindrical) materials placed in a uniform alternating magnetic field, the power absorbed by the particle P_(w) can be:

$\begin{matrix} {P_{w} = {{\frac{\rho}{\delta}AKH^{2}} = {AKH^{2}\sqrt{\pi f\mu\rho}}}} & \lbrack 2\rbrack \end{matrix}$

Where ρ is the resistivity of the material, δ is the skin depth, A is the particle surface area exposed to the magnetic field, K is a power transfer factor that depends on particle geometry, and H is the magnetic field strength. It should be noted that resistivity changes as a function of temperature and, as such, it is envisioned that the P_(w) may be adjusted through time depending upon changes in static and dynamic thermal conditions during the formation of a component. It is possible to calculate the power absorbed by a given metallic particle in an induction heating process using modern finite element analysis methods. As a rule of thumb, with a fixed resistivity, magnetic permeability and particle dimensions, the power absorbed by the particle in an induction heating process increases with increasing frequency and magnetic field strength.

The only ill-defined quantities are A and K, which describe how well the high frequency magnetic field couples to the individual particle. For any given slice through an approximately spherical particle, d/δ can be calculated from the particle diameter at that slice. The power transfer factor K, on the other hand, depends on the “electrical dimension” of the portion of the particle being heated, which is defined as the ratio of the diameter of the particle to the skin depth, d/δ.

FIG. 3 represents a graph representing power transfer factors for materials subjected to micro induction heating. Power transfer factors for two cases of a plate and a cylinder are shown. Using the plate geometry as a crude model for roughly spherical particles, it is seen that K approaches unity if the skin depth is much smaller than the thickness of the particle. For example, when d˜26δ K is approximately 0.8. The system utilizes the functional dependence of K(d/δ) for determining the appropriate frequency or frequencies for the selective heating of individual particles in composite material. The system 10 utilizes two conceptual composite architectures with an emphasis on the selective heating of individual components of the composite component during the consolidation process. Accordingly, the selectivity of the system's micro-inductive sintering is based both on the size and material properties of the particles in the powder.

FIGS. 4a-4c illustrate the application of micro-inductive sintering to a mixture of two mono-sized dispersed metal powders. In FIG. 4a , the powder mixture 12 consists of a first material 14 and a second material 16 with approximately the same particle size or particle size distributions, but with different material properties. The resistivity p of the particles of the first material 14 is ten times greater than the resistivity of particles of the second material 16. Assuming that bulk heating of the particles occurs when 2 δ is equal to the diameter of the particle, the induction frequency can be:

$f = \frac{4\rho}{\pi\mu d^{2}}$

where d is the diameter of the particle.

Thus, for a given particle size and magnetic permeability, the induction frequency to achieve bulk heating of a particle scales linearly with the resistivity of the material. In this case, the particles of the first material 14 can be selectively heated in bulk using an oscillating magnetic field with a frequency ten times smaller than that which would be used to bulk heat the particles of the second material 16. This is illustrated in FIG. 4b , which explicitly shows the selective bulk heating of the particles of the first material 14 as indicated by double cross-hatching. FIG. 4b depicts the heating of the particles of the first material 14 where the frequency of the magnetic field is set such that the skin depth is approximately one half the diameter of the particle. The skin depth of the particles of the second material 16 is approximately (10)^(0.5)˜3.2 times that of the first particle at this frequency. Since the skin depth in the second particle is much larger than the particle diameter, there is very poor coupling to the high frequency magnetic field and these particles are not heated directly by induction. Note that the particles of the second material 16 are also heated in this process, but only by conduction and convection heating which results from the induction heating of the particles of the first material 14. As such, only an outer portion 18 of the particles of the second material 16 are heated as depicted by double cross-hatching. The selective sintering of powders that possess similar particle size distributions, but different materials properties can be used to inform the power levels and frequencies needed for micro-inductive sintering.

FIG. 4c represents a portion of a consolidated component 11 where the previously heated and melted particles of the first material 14 have now cooled after completion of the selective sintering process. It should be noted that the isolated particles of the second material 16 remain as inclusions within the recently formed solid of the first material 14. Upon consolidation of the particles of the first material 14, the effective domain size of the first material 14 increases such that the high frequency magnetic field tuned to the initial diameter of the particles of the first material 14 no longer couples well to the first material 14. In this case, the effective particle size is much larger than the skin depth at this frequency and the entire consolidated domain is heated only at the surface as previously described in relation to FIG. 4 b.

In one exemplary manufacturing method, the bed 13 of the mixture of powder 12 may be heated to a temperature near the melting temperature of the particles of the first material 14. Only the very low overall additional energy needed to melt the powder 12 need be inputted into the powder bed 13 by the flux concentrator 17 to selectively melt the particles of first material 14. The additional energy is localized to the active micro-inductive sintering zone near a gap 23 in the flux concentrator 17. For example, high frequency induction of eddy currents in a metallic binder (particles of the first material 14) allows for the selective heating and subsequent consolidation of a ceramic/metal matrix composite without the associated heating and degradation of the ceramic constituent (particles of the second material 16). This makes it possible to consolidate composites composed of very heat-sensitive ceramic particles (e.g., superconducting materials).

The coupling and de-coupling of the high frequency magnetic field based on the domain size of the metallic material is a unique and novel feature specific to the micro-inductive sintering process of the present disclosure. This property allows for real-time diagnostics of the micro-inductive sintering consolidation process through the monitoring of the forward and reflected power to the powder bed. In addition, this process allows for the rapid and automatic de-coupling of the external heat source (i.e. the high frequency magnetic field) upon consolidation of the particles. This is a desirable control feature in the consolidation of heat sensitive materials or composite materials that may degrade upon exposure to elevated temperatures.

As previously stated, the selectivity of the system's micro-inductive sintering is based both on the size and material properties of the particles in the powder. The metal powder shown in FIG. 5a consists of a bimodal distribution of first particles 22 and second particles 24. The second particles 24 are the larger of the two particles having approximately twice the diameter of the smaller first particles 22. Again, either the smaller or larger particles may be selectively heated by the induction frequency, where it is seen that the induction frequency varies as a function of size. Thus, a twofold increase in particle size implies a fourfold decrease in the frequency of the oscillating magnetic field necessary to achieve bulk heating, assuming the optimum “electrical dimension” for heating the particles was equal to 2.

FIG. 5b illustrates the bulk heating of the smaller first particles 22 and the surface heating of an outer portion 26 of larger second particles 24 which is characteristic of the micro-inductive sintering process. Using a narrow bandwidth of fixed frequencies, complete consolidation of the effected region is shown in FIG. 5c . As in the previous example, upon consolidation of the particles, the effective domain size of the material increases and the high frequency magnetic field tuned to the initial diameter of the smaller first particles 22 becomes de-coupled from the consolidated material and the entire domain is heated by induction only at the surface.

In the composite architectures previously described, the frequency of the induction heating process is used to selectively heat specific components of the composite based on the physical or materials characteristics of the powder. In the prior example, the small first particles 22 are selectively heated by induction, which results in the consolidation of the material. By changing the frequency or spectrum of the magnetic field, however, the large particles could have been selectively heated by induction, which may lead to an improved density of the final part. In practice, the specific sintering characteristics of the material and the desired material properties of resultant material will determine the micro-inductive sintering frequency spectrum. Overall, the micro-inductive sintering approach allows for enhanced control of the densification process by targeting small particles, or large particles that can be partially or entirely melted. This control adds another tool in the toolbox for the effective consolidation of powders suitable for use in additive manufacturing.

By selective application of the magnetic fields, micro induction sintering produces complex parts and components directly from advanced metal and ceramic/metal matrix composite powders. The micro-inductive sintering process, however, is not without limitations imposed by the system electronics, the magnetic properties of the magneto-dielectric material used to fabricate the flux concentrator 17, the specific sintering characteristics of the metallic powders, and the fundamental physics of induction heating. In general, the micro-inductive sintering processing is preferable within the following operational parameters:

1) Materials with electrical resistivity between 1 μOhm cm and 200 μOhm cm.

2) Powders with particle sizes between 1 μm and 400 μm.

3) Flux concentrator induction frequencies between 1 MHz and 2000 MHz.

The operative phase space for the bulk and surface heating of powders by high frequency induction can be determined. FIGS. 6a and 6b show the operative phase space for the micro-inductive sintering process with a fixed particle diameter of 100 μm (FIG. 6a ) and with a fixed resistivity (FIG. 6b ) assuming an “electrical dimension” of 2. The highlighted area between the surface and bulk heating curves indicates the parameter space in which the micro-inductive sintering process operates. Any material that falls out of this range is not a preferred candidate for the micro-inductive sintering process based on the operational parameters listed above. An alternative representation is shown in FIG. 6c , which illustrates the operational frequency of the micro induction sintering process as a function of particle size and resistivity. There are three primary operational frequency bands shown in the Figure:

High Frequency (HF)—frequencies less than 30 MHz and greater than 0.1 MHz.

Very High Frequency (VHF)—frequencies greater than 30 MHz and less than 300 MHz.

Ultra High Frequency (UHF)—frequencies greater than 300 MHz and less than 3 GHz.

The vast majority of materials used in additive manufacturing processes possess particle size distributions ranging between 50 μm and 150 μm with electrical resistivities less than 100μΩ cm. This operational space is highlighted by the box outline in FIG. 6c , which shows that most materials can be heated by the MIS process in the VHF and UHF bands. A material that falls within the lowermost, non-hatched region shown in FIG. 6c may not be a practical candidate for the MIS process based on the operational parameters listed above.

One example of a material formed using the micro-inductive sintering system is Tungsten Carbide/Cobalt (WC—Co) composites which are used extensively for machine tools, metal cutting, dies, and wear resistant coatings. These materials are fabricated with a fine dispersion of WC particles in a Co matrix (5% to approximately 30% by weight) and demonstrate high strength, high toughness and high hardness. WC—Co parts are conventionally formed using powder metallurgy and sintering at approximately 1700K. Cobalt serves as the binding metal in these composites and a uniform dispersion is critical to the overall performance of the composite material.

According to the present teachings, a WC and Co composite of powdered material is formed with average component domain sizes of 1 μm and 5 μm, respectively. This powder has morphology similar to that shown in FIG. 5a , with the smaller first particles 22 being WC and the larger second particles 24 being Co metal. The materials properties of WC and Co relevant to the micro-inductive sintering process are shown in Table 1. In order to consolidate this material by high frequency induction, the cobalt matrix is selectively melted, which will then cause densification of the composite through liquid phase sintering. As seen in Table 1, cobalt is magnetic with a relative permeability on the order of 100 and a Curie temperature of approximately 1400K. Thus, heating of the Co particles will occur due to Joule heating and hysteresis loss at temperatures less than the Curie temperature. Above the Curie temperature, the only heating mechanism is via eddy currents in the metallic domains. The Micro-Induction Sintering of WC—Co composites are a complicated but feasible process involving a wide bandwidth, high-frequency, flux density that is primarily coupled to the cobalt metal.

During the micro-inductive sintering consolidation process, the effective domain size of the cobalt increases which, as discussed previously, will de-couple the high-frequency magnetic field from the material. The WC particles, on the other hand, do not change in size during the process. Additionally, the size and electrical characteristics of WC are not effectively heated directly by the high-frequency magnetic field. Table 2 shows calculations for the bulk heating frequency of WC and Co particles as a function of temperature and domain size.

The high resistivity of WC prevents this material from being heated by induction. A 5 μm WC particle, for example, would require an induction frequency of approximately 8 GHz for bulk heating. Although envisioned herein, these frequencies are impractical given the present state of existing flux concentrator materials and power supplies. For micro-inductive sintering process frequencies less than 100 MHz, the WC particles are magnetically “invisible” to the high frequency magnetic field and are only heated indirectly through contact with the cobalt.

Based on predictive performance within the envelope of the operational parameter phase space shown in FIGS. 6a, 6b, and 6c , Tungsten Carbide/Cobalt is a good candidate for micro-inductive sintering-based particle formation. The strength, toughness, and hardness of WC—Co composites make the material highly desirable for many manufacturing applications.

It may also be desirable to construct components as fiber reinforced metal composites. Fiber reinforced metal composites are made from a variety of materials including carbon, silicon carbide and alumina fibers in a metal matrix, typically aluminum, titanium or magnesium. The addition of the fibers can dramatically improve the performance of the metal by increasing strength, stiffness, dimensional stability and heat resistance. The defense, automotive, and aerospace markets have a strong interest in these materials in order to deliver higher performance, higher reliability and/or lighter weight components. Key applications areas for these materials today include engine cylinder blocks and pistons, brakes, and dimensionally stable structural components for use in space.

The micro-inductive sintering process can provide an improved method for consolidating carbon fiber-reinforced composites, particularly for aluminum and magnesium-based matrix materials. Tables 3 and 4 show the bulk heating frequency of Al, Mg, and representative carbon fiber as a function of particle size. Since the micro-inductive sintering process is based on powder metallurgy, the challenge of getting a homogeneous distribution of the carbon fibers during the liquid phase infiltration step is eliminated. As shown in FIGS. 7a-7c , to produce these materials the metal powder and carbon fibers are combined using conventional powder milling methods to give a uniform distribution of the carbon fiber reinforcement with the metal matrix.

A further advantage of the micro-inductive sintering process is that it allows for the selective heating and rapid cooling of the metal matrix. As discussed previously, the induction frequency spectrum can be set such that the skin depth is approximately one-half of the particle size distribution of the metal matrix. Upon consolidation of the metal particles, the effective domain size increases and the micro-inductive sintering frequency spectrum will no longer couple well to the larger metallic domain. In a sense, this heating process is self-limiting in that it becomes inefficient as soon as the particles in the metal matrix coalesce. Aluminum and Al—Mg alloys possess relatively low melting points, which in combination with the very high vapor pressure of Mg, can lead to the loss of material during the additive manufacturing process if the local temperature is too high. The materials properties of Al, Mg, and representative carbon fiber relevant to the micro-inductive sintering process are shown in Table 3.

The high vapor pressure of Mg near and above its melting point can lead to a significant change in the composition of high strength Al—Mg fiber reinforced alloy parts fabricated using alternative additive manufacturing technologies.

The Micro-Induction Sintering of carbon fiber-composites is characterized by a wide bandwidth, high-frequency, flux density that is coupled to the metal matrix. During the micro-inductive sintering consolidation process, the effective domain size of the metal matrix increases which, as discussed previously, will de-couple the high frequency magnetic field from the material. The high resistivity of carbon, along with the small dimensions of the fibers, render the carbon fibers invisible to the induction heating process at frequencies less than 100 MHz. The induction frequency for aluminum and magnesium particles with the appropriate particle size distribution is between 2 MHz and 20 MHz. Similar to the WC particles in WC—Co composites, the carbon fibers are heated only through the contact with the metal matrix.

The composite starts out as a homogeneous mixture of powder metal matrix (the first material 14) and reinforcing fibers (the second material 16). The matrix material is heated to sintering temperatures by a high-frequency magnetic field tuned to match the particle size of the metal powder as shown in FIG. 7b as indicated by double cross-hatching. Upon consolidation, the metal matrix decouples from the oscillating magnetic field and rapidly cools. The resulting composite is a consolidated material with uniform distribution of reinforcing fibers as shown in FIG. 7 c.

As shown in FIG. 8, the carbon reinforcement fibers can take on a variety of forms from single walled carbon nanotubes with a diameter of 1 nm, to multi-walled carbon nanotubes with diameters of 10-100 nm to carbon fibers with diameters of 1-10 μm. Table 5 shows a potential list of candidate materials/applications. While certain of the candidate materials considered were eliminated due to technical suitability for the micro-inductive sintering system, some were eliminated simply because other manufacturing methods exist for the materials and thus should be considered for the applications of the techniques disclosed herein. It is envisioned the techniques described herein may be used with all of these materials.

It is expected that the micro-inductive sintering system will show advantages over laser system with reflective metals (e.g. aluminum and niobium) as the optical reflection of the laser from the metal drives a need for much higher powered lasers for sufficient heat absorption to consolidate the metal. Optical reflectivity has no effect on the micro-inductive sintering system's interaction with materials, and therefore the micro-inductive sintering approach may prove to be more efficient with these materials.

In addition to continued exploration of the chosen materials, it is expected that new material compositions will be developed which are either targeted for use with the micro-inductive sintering method (specifically to take advantage of the unique capabilities of the process) or that naturally have benefit from the techniques and methods which will be described in detail below.

FIG. 9 represents the simulated application of high frequency magnetic fields to the mixture of powder 12. As shown, the mixture of powder 12 includes the particles of the first material 14 having a first particle size distribution 30 and the particles of the second material having a second particle size distribution 32. Corresponding to the first and second particle size distributions 30, 32 are a pair of magnetic field frequency distributions 34, 36. The magnetic field frequency distributions 34, 36 correspond to a group of frequencies designed to melt the particles of the first and second materials.

As described above, the system 10 can apply a frequency spectrum 37 of magnetic fields ranging between vertical lines A and B or a frequency spectrum 38 ranging between lines B and C to melt only one of the two materials. Alternatively, the system 10 can apply a spectrum 39 ranging between lines A and C configured to melt a majority of both the particle distributions. The system can sequentially apply the magnetic field through the flux concentrator 17 as a frequency sweep. Alternatively, the signals to the flux concentrator 17 can be multiplexed and can include all or a portion of the frequencies within a spectrum.

FIG. 10 includes a flow chart describing the formation of a bi-material component 11 according to the present teachings and depicted in FIGS. 11a-11c . The method for forming a component 11 includes producing a layer of powder mixture 12. Next, a magnetic field is applied to a first portion 40 of a first layer 42 of the powder mixture 12 to selectively melt the first portion 40 of the powder mixture 12. Preferably, the powder mixture 12 is formed of particles of the first and second materials 14, 16 having mean diameters between 1 μm and 400 μm, and resistivity between 1 μΩ cm and 200 μΩ cm. Based on the size distribution and resistivity of the particles of the first material 14, the absorbed power changes the phase of the effected particles of the first material to liquid. The first liquid material is solidified automatically (FIG. 11b ) as the melted particles no longer exhibit the proper dimensions for bulk heating by the applied magnetic field. Alternately, solidification may be achieved by removing the magnetic field.

As shown in FIG. 11c , after the portion 40 of the first layer 42 of the powder mixture 12 has been solidified, a second layer 46 of mixture is disposed over the first layer 42. Second portions 48 a and 48 b of the mixture are then melted and solidified using the concentrated magnetic field. Layers of mixtures are recursively added and sintered to form the component. Optionally, the system 10 can apply a magnetic field, having a frequency between about 1 MHz and about 2.0 GHz, to the first layer of the powder mixture to melt at least a skin layer of a portion of the first material.

FIG. 12 is a schematic representation of flux concentrator 17 shown in FIG. 1. The flux concentrator 17 can have a horse-shoe shape having a curved body 50 which supports an inductive coil (not shown). Tapered arms 52 extend from curved body 50 to concentrate the magnetic field at a centralized location namely, gap 23.

Example embodiments are provided so that this disclosure will be thorough, and will fully convey the scope to those who are skilled in the art. Numerous specific details are set forth such as examples of specific components, devices, and methods, to provide a thorough understanding of embodiments of the present disclosure. It will be apparent to those skilled in the art that specific details need not be employed, that example embodiments may be embodied in many different forms and that neither should be construed to limit the scope of the disclosure. In some example embodiments, well-known processes, well-known device structures, and well-known technologies are not described in detail.

The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limiting. As used herein, the singular forms “a,” “an,” and “the” may be intended to include the plural forms as well, unless the context clearly indicates otherwise. The terms “comprises,” “comprising,” “including,” and “having,” are inclusive and therefore specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. The method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed.

When an element or layer is referred to as being “on,” “engaged to,” “connected to,” or “coupled to” another element or layer, it may be directly on, engaged, connected or coupled to the other element or layer, or intervening elements or layers may be present. In contrast, when an element is referred to as being “directly on,” “directly engaged to,” “directly connected to,” or “directly coupled to” another element or layer, there may be no intervening elements or layers present. Other words used to describe the relationship between elements should be interpreted in a like fashion (e.g., “between” versus “directly between,” “adjacent” versus “directly adjacent,” etc.). As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.

Although the terms first, second, third, etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms may be only used to distinguish one element, component, region, layer or section from another region, layer or section. Terms such as “first,” “second,” and other numerical terms when used herein do not imply a sequence or order unless clearly indicated by the context. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the example embodiments.

Spatially relative terms, such as “inner,” “outer,” “beneath,” “below,” “lower,” “above,” “upper,” and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. Spatially relative terms may be intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, the example term “below” can encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly.

The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be regarded as a departure from the disclosure, and all such modifications are intended to be included within the scope of the disclosure. 

What is claimed is:
 1. A method for forming a component comprising: providing a first layer of a mixture of first and second powders; determining a frequency of an alternating magnetic field to induce eddy currents sufficient to (i) bulk heat only the first powder and bulk heating of particles of the first powder occurs when a diameter of each of a plurality of particles of the first powder is at least equal to 2δ for the respective particle of the first powder, wherein: $f = \frac{4\rho}{\pi\mu d^{2}}$ where d is the diameter of a respective particle of the first powder, ρ is the resistivity of the material of a respective particle of the first powder, f is the frequency of the alternating magnetic field, and μ is the magnetic permeability of the material of the respective particle of the first powder and (ii) surface heat particles of the second powder; and applying the alternating magnetic field at the determined frequency to the first layer of the mixture using a flux concentrator, wherein exposure to the magnetic field changes the phase of at least a portion of the first powder to liquid by bulk heating the particles of the first powder and heating the particles of the second powder such that outer regions of the particles of the second powder change phase to liquid while inner regions of the particles of the second powder remain solid.
 2. The method of claim 1, wherein two or more particles of the first powder combine to form a consolidated material after the two or more particles change to the liquid phase, the consolidated material having a size de-coupling the magnetic field from the consolidated material.
 3. The method of claim 1, wherein surface heating of each particle of the second powder occurs when 2δ for the respective particle of the second powder is less than a diameter of the respective particle of the second powder.
 4. The method of claim 1, wherein determining the frequency of the alternating magnetic field is based on a dimension of the particles within the first powder.
 5. The method according to claim 4, wherein the first powder has a first mean particle diameter and the second powder has a second mean particle diameter which is different from the first mean particle diameter.
 6. The method of claim 1, wherein determining the frequency of the alternating magnetic field is based on a resistivity of particles with the first powder.
 7. The method according to claim 6, wherein the first material has a first resistivity and the second material has a second resistivity different than the first resistivity.
 8. The method according to claim 1, wherein the first powder includes particles from a first material and the second powder includes particles from a different second material.
 9. The method according to claim 8, wherein the first material is a metal and the second material is carbon fiber.
 10. The method according to claim 8, wherein the first material is WC and the second material is cobalt.
 11. The method according to claim 8, wherein the first material is a metal and the second material is a ceramic.
 12. The method according to claim 1, further comprising applying a second layer of powder material over the first layer of powder material and applying the magnetic field to the second layer.
 13. The method according to claim 1, further comprising ceasing to apply the magnetic field to solidify the liquid portion and couple a portion of the first powder with portions of the second powder. 